1. Field of Invention
The present invention relates to a combination door lock, and more particularly to a self-canceling combination mechanism which displaces the position of the combination discs inside the lock after the bolt has been retracted.
2. Description of Prior Art
In U.S. Pat. No. 4,328,689, which is assigned to the Applicant of the present invention, there is described a combination lock with a spring-acutated bolt. The bolt is retracted by positioning a plurality of combination discs, located inside the lock, to a predetermined position, and this is done by rotating a combination dial outside a door on the inside of which the lock housing is secured. In order to retract the bolt, as soon as the combination of the dial has been completed, a plurality of notches in the combination disc are all aligned and a retracting lever support pin enters these notches causing the retracting lever to engage a cam which is secured to the combination dial. By rotating the dial the bolt retracting lever is pulled, and the bolt can be pulled back into the housing and the door opened. With such a combination lock a person may become knowledgeable that once the door has been opened, all of the notches in the combination discs are aligned even after the door has been re-closed, provided the dial has not been rotated to scramble the combination. Thus, if one forgets to scramble the combination, then it only requires easy manipulation of the combination dial by an expert thief to reset the combination.
Another disadvantage of the type of prior art locks as mentioned hereinabove, where the bolt is spring-actuated, is that the bolt can be pushed back into the lock housing by drilling a hole through the doorframe in the direction of the bolt and then inserting a pin element in the hole to push the bolt back into the lock housing.